Generally speaking, in the field of direct current discharge lamps for use in a light treatment such as, of forming an electric circuit onto a semiconductor or onto a printed board, various devices have been suggested for making a series of actions from lighting of the discharge lamp up to stable running of the discharge lamp smooth. As a typical example of such a type of discharge lamp, a so-called short arc type discharge lamp whose distance between electrodes is several mm will be described.
When started, the short arc type discharge lamp causes dielectric breakdown of a sealed gas to reach an arc discharge via a glow discharge, and then stabilizes the arc discharge so as to ensure stable lighting of the discharge lamp. FIG. 9A shows the curves showing the conditions that the voltage and current are changed from the dielectric breakdown to the state of retaining the arc discharge.
As shown in FIG. 9A by an arrow (11) (see curve A), the sealed gas such as mercury or xenon is ionized by the application high voltage, typically of 1000.degree. C. or more, to the electrodes of the discharge lamp to generate an electrically conductive gas. This phenomenon is called "dielectric breakdown". Subsequent continuous application of a voltage over several hundreds volt to the electrodes causes the collision of the electrons with the atoms in the ionized, electrically conductive gas, leading to the further progress of the ionization. This results in the retaining of the discharge (see arrow (12)). The electrons taking part in the ionization are those discharged from the atoms, as a result of the dielectric breakdown of the gas. The state that the discharge is continued is called "glow discharge". In this case, the voltage between the electrodes at which the discharge is generated is generally from approximately 90 to 120 V, and the current run between these electrodes is not more than several hundreds mA.
When the current run continuously through the gas in the glow discharge state, due to the electrode being heated by the electron collision, the amount of electrons thermally discharged from the electrode and, at the same time, the ionization of the sealed gas is promoted to become a conducting state. This state is called "arc discharge" (see arrow (13)). In this state, the voltage to flow between the electrodes are generally from 20 to 60 V, and it becomes easier for the current of from several A to several hundreds A between the electrodes.
As devices for lighting such a discharge lamp, various types of the devices have been suggested. For examples, there exist a lighting device comprising a resistor so-called leakage resistor capable of regulating the current at the time of shorting the load, and a switch mode lighting device utilizing semiconductor elements, and a lighting device which utilizes a starter for causing the dielectric breakdown of the sealed gas within the discharge lamp to cause the glow discharge and the arc discharge.
The starter of the lighting device is composed of a direct current high voltage generating circuits, which arranged on the discharge lamp in parallel and, at the same time, diode connected between the main power circuit and the discharge lamp not so as to apply a high voltage to the direct current main power circuit for maintaining the arc discharge.
The starter for the discharge lamp may be on which can generate a voltage of not less than 1000 V, which is required for the dielectric breakdown of the sealed gas, and which can supply a current enough for starting the arc discharge. The main power may be one which can supply a current and a voltage enough for maintaining the arc discharge as shown on the curve d of FIG. 9A. In this context, the output current and power characteristics of the starter should be those shown as the dot line b of FIG. 9A.
However, since it is required to generate a high voltage enough for the dielectric breakdown, the starter utilizes a transformer having a high boosting ratio. For this reason, the starter can only supply a faint current and, thus, the output voltage and current characteristics of the discharge lamp results in a double broken line b of FIG. 9A. Consequently, the dielectric breakdown of the sealed gas can be caused, but it becomes sometimes impossible to maintain the glow discharge state and to make transition transferred to the arc discharge.
In order to solve such a problem, a device for lighting a discharge lamp has been disclosed in which a smoothing capacitor for rectification having a large capacity so that electric charge collected in the capacitor are instantly run through the discharged lamp at the time of starting the discharge, and by the virtue of the large amount of the electric current at this time, the state is shifted to the arc discharged state (see Japanese Patent No. 2705010 and Japanese Patent No. 2705018).
However, even if the devices are made as described above, there sometimes causes the dielectric breakdown at a relatively low voltage of approximately 500 V due to an influence of a temperature, etc., upon the discharge lamp. In such a case, since the voltage, charged in the smoothing capacitor, is as low as 500 V, it becomes impossible to make transition into the arc discharge. In order to solve such a problem, a method has been suggested in which a switching element such as an arrester, which conducts when the voltage reaches a prescribed level such as 1000 V, is connected between the smoothing capacitor and the discharge lamp. This method is applied to the construction of the circuit well known as relaxation oscillation circuit utilizing a non-linear switching element such as diode AC switch (DIAC).
However, the conventional devices for lighting a discharge lamp have the following problems:
(1) In the case of using a relaxation oscillation circuit, the switching element becomes conductive at the first time when the voltage becomes high so that the energy of the smoothing capacitor can be instantly supplied to the discharge lamp. For this reason, the voltage and the electric current applied to the discharge lamp are very rapidly changed in comparison with the normal relaxation oscillation circuit. The wave form thereof includes portions at which big oscillation occurs as shown in FIG. 9B. This causes radio frequency interference to malfunction of power circuit.
(2) In this context, it has been carried out that an inductance element is inserted into the discharge lamp in series to relax the oscillation current. However, due to relatively high inductance required, any closed magnetic circuit cannot be used for preventing magnetic saturation against a large amount of current, which runs instantly, and thus, the inductance element used must be very large size, inducing the problem in terms of heavy weight.
(3) As for the starting method utilizing a smoothing capacitor and discharge, due to very large current running from the smoothing capacitor to the discharge lamp becomes very large (although the period thereof is short), the sputtering of electrode is caused, making the service life markedly short.
(4) When the power source connected to the discharge lamp, which can supply the voltage and the current required for maintaining the glow discharge and the arc discharge, is used, it is possible to be shifted into the arc discharge state immediately after the dielectric breakdown of the sealed gas. For maintaining the glow discharge state, although a voltage required is 1000 V or more, the current required may be of relatively small amount. On the other hand, in order to maintain the arc discharge state, the voltage required is only 60 V, but a current in an amount of 1 A or more should be required, and in some cases, a power source which can run a current exceeding 100 A is required. Consequently, if single power circuit is used for maintaining both the glow discharge and the arc discharge, due to large heat loss in the power circuit in the state of the arc discharge, the power source having unduly large capacity must be used.
(5) In the case where the discharge lamp cannot be turned on, for example, under the influence of a temperature, the dielectric breakdown at a lower voltage occurs, and then the discharge lamp cannot be shifted into the glow discharge, or in the case when the turn on is tried soon after the turn off of the lamp, it is again turned on, there is a disadvantage that the starter is not restarted until the voltage applied to the discharge lamp is very high.